1. Field of the Invention
The present invention relates to a flow rate control valve capable of opening/closing a fluid circuit and freely adjusting the opening degree thereof by effecting cooperation between an expandable material and a flexible thin film.
2. Description of the Related Art
A flow rate control valve, which is based on the system called xe2x80x9cthermal systemxe2x80x9d or xe2x80x9cheat systemxe2x80x9d and which utilizes a thermally expandable material, has been hitherto widely known. Reference may be made, for example, to Japanese Patent Publication No. 2708395 (U.S. Pat. No. 4,824,073).
As shown in FIG. 11, the conventional flow rate control valve comprises a silicon wafer 8 which has a flexible wall 4, which includes a certain amount of substance 2, and which has a thin film chamber 1 for accommodating the substance 2. The thin film chamber 1 is formed of at least the silicon wafer 8. The thin film chamber 1 serves as a hermetic vessel to capture the substance 2. The flow rate control valve further comprises a pyrex wafer 7 which has a heating means 3 facing the thin film chamber 1. The substance 2 is closed by an epoxy cap 6.
The conventional flow rate control valve has a fluid flow passage 10 including an internal bore which is formed in a pyrex wafer 9 connected to the silicon wafer 8, which is disposed adjacently to the flexible wall 4, and which is constructed so that a fluid to be controlled flows therethrough. The bore is formed by a space between the flexible wall 4 and a sealing surface 5 formed on the pyrex wafer 9. The flow rate of the fluid flowing through the fluid flow passage 10 is controlled by the flexible wall 4 in cooperation with the sealing surface 5.
The heating means 3, which is provided on the pyrex wafer 7, heats the substance 2 so that the bending amount of the flexible wall 4 is controlled to control the cross-sectional area formed by the gap between the flexible wall 4 of the silicon wafer 8 and the sealing surface 5 formed on the pyrex wafer 9. Thus, the flow rate of the fluid flowing through the fluid flow passage 10 is controlled.
Japanese Laid-Open Patent Publication No. 5-233068 discloses a system for controlling the flow rate of a fluid flowing through a flow passage by providing, as means for detecting the flow rate of the fluid, pressure gauges disposed on the outlet of a valve mechanism section of a mass flow controller.
That is, as shown in FIG. 12, a fixed orifice 13 is provided for a gas flow passage 14. A first pressure gauge 15, which measures the pressure by using the change in capacity or volume of a diaphragm that is deformable depending on the change in pressure, is provided for the fixed orifice 13. A second pressure gauge 16 is provided upstream from the fixed orifice 13. The differential pressure between the pressures measured by the first and second pressure gauges 15, 16 is converted into a flow rate which is outputted as a signal.
The output signal (detection signal) is compared with a preset signal by using a comparing circuit 12. A variable valve 17 is controlled by means of a control circuit 11 so that the difference between the output signal and the preset signal is zero to control the flow rate of the fluid flowing through the gas flow passage 14.
According to the technical concept disclosed in Japanese Patent Publication No. 2708395, however, if any deterioration or any time-dependent change is caused, for example, as a result of the use for a long term in the flexible wall 4 and the substance 2 which is expandable by being heated by the heating means 3, then it is impossible to correctly grasp the displacement amounts of the substance 2 and the flexible wall 4, and the controlled flow rate of the fluid becomes unstable.
For this drawback, it is assumed that an unillustrated temperature sensor is provided for the substance 2 and the flexible wall 4 in order that the displacement states of the substance 2 and the flexible wall 4 are grasped to stabilize the flow rate of the fluid and avoid any overheating or any insufficient heating effected by the heating means 3.
However, the temperature sensor is affected by the ambient temperature at which the flow rate control valve is used, and it is impossible to obtain no sufficient accuracy for correctly grasping the displacement amounts of the substance 2 and the flexible wall 4.
According to the technical concept disclosed in Japanese Laid-Open Patent Publication No. 5-233068, the pressure for the flow rate of the fluid is detected by the pair of pressure gauges 15, 16, and the detection signal is fed back to the control circuit 11 for the variable valve 17 to open/close the variable valve 17 and adjust the opening degree thereof in order that the flow rate of the fluid is stabilized. However, the response is slow, and the overshoot or the undershoot is apt to occur, because the feedback loop is large.
A general object of the present invention is to provide a flow rate control valve capable of improving the response performance for controlling the flow rate of a fluid flowing through a fluid flow passage and the stability of the flow rate of the fluid.
The flow rate control valve according to the present invention comprises at least one pressure-detecting sensor which detects a pressure brought about when an expandable material is expanded by the heating action of a heating mechanism. A control signal is outputted by a control mechanism to the heating mechanism based upon a detection signal derived from the pressure-detecting sensor to adjust the spacing distance between a sealing section and a flexible thin film which is flexibly bendable in cooperation with the expanding action of the expandable material, or adjust the spacing distance between a sealing section and a tiltable member which is tiltable by a flexible thin film which is flexibly bendable in cooperation with the expanding action of the expandable material.
As a result, in the present invention, an obtained feedback loop is smaller than the conventional feedback loop (see FIG. 12). The output response is improved for the control signal controlled by the control mechanism based upon the detection signal. Accordingly, the occurrence of the overshoot or the undershoot is suppressed. Thus, it is possible to further stabilize the flow rate of the fluid flowing through the fluid flow passage.
According to the present invention, the displacement state of the flexible thin film which is flexibly bendable in cooperation with the expandable material is detected by the pressure-detecting sensor in order to avoid the overheating or the insufficient heating of the heating mechanism. Therefore, no influence is exerted by the ambient temperature at which the flow rate control valve is used.
According to the present invention, the time-dependent change and the deterioration state can be also grasped by previously storing, in an unillustrated memory means, the initial value of the pressure brought about when the expandable material is expanded, and comparing the initial value with an actually measured value obtained when the flow rate control valve is used.
According to the present invention, the unillustrated memory means previously stores the flexibly bending displacement amount of the flexible thin film depending on the pressure brought about when the expandable material is expanded and the displacement amount of the tiltable member which is tilted by the flexible thin film. Thus, the self-diagnosis can be made for the positional state of the flexible thin film which is flexibly bent in cooperation with the expandable material and the tiltable member which is tilted by the flexible thin film, regardless of whether or not the fluid flowing through the fluid flow passage exists.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.